The present invention relates to electronically conductive polymers, as well as formulations and methods for preparing electronically conductive polymers. More particularly, the present invention relates to electronically conductive polymers having enhanced electrical conductivity.
The trend toward miniaturization, integration and automated assembly in the electronics industry is forcing designers to continually increase the component density in integrated circuit manufacturing, interconnection and packaging. Current demand for increasingly complex printed wiring boards (PWBs) has resulted in increasingly stringent requirements for all steps in their production. To produce high-quality boards at competitive prices requires that production costs be kept down. Lower production costs can be achieved by using and producing lower quantities of environmentally toxic chemicals, reducing the number of manufacturing steps, shortening process times, and increasing the degree of automation.
The introduction of double-sided boards, followed by multilayer boards, was made possible by metallizing plated through-holes with electroless copper. For the last 25 years, 98 percent of the PWBs manufactured have used this technology. However, electroless deposition of copper requires a potent reducing agent, such as formaldehyde, which is a reported carcinogen. Most electroless copper solutions also contain cyanide and chelating agents, which are difficult to remove from waste streams. Besides the normal drag-out associated with wet processing, xe2x80x9cbail-outxe2x80x9d (required to maintain solution balance and periodic bath changes) renders waste treatment of electroless copper far more expensive than electroplated copper. Another environmental and waste treatment concern associated with electroless copper is that it requires stripping copper from racks and tanks with nitric acid.
Electroless copper deposition typically involves a seven-step process with interval rinses with water that become contaminated with copper sulfate/EDTA/formaldehyde bath components. During electroless copper deposition or metallization, copper metal is deposited over the entire board surface and sensitized walls of through-holes, usually to a thickness of 0.001 in. Environmental concerns associated with electroless copper metallization, have fostered interest in direct metallization processes. Despite numerous attempts over the last 10 years, conversion to direct metallization processes has not gained widespread acceptance, and only about five percent of PWB manufacturers worldwide have eliminated metallization by electroless copper.
In addition, electronics manufacturers have not realized or appreciated the benefits that direct metallization can provide. These include reduced waste treatment/processing costs, lower chemical costs, improved efficiency/reliability, and the elimination of a time-consuming procedure.
Electronically conducting polymers have often been categorized as non-processable and intractable, because of their insolubility in the conducting form. Only recently has it been shown that polymers such as polyaniline can be dissolved using functionalized sulfonic acids. For polypyrrole, this can be achieved by using its derivatives [e.g., poly (3-octylpyrrole)] which are known to be soluble in different solvents, or by treatment in dilute aqueous sodium hypochlorite solutions, ammonia or mono-, di- or tri-substituted amine (co)solvents. Another method of solubilizing polypyrrole is the process of polypyrrole chain deprotonation in basic solutions, which causes a transformation of conducting polypyrrole into a non-conducting polymer of quinoid structure.
The lack of processability of conducting polymer materials, e.g., solution or melt processing, infusability and poor mechanical properties, e.g., ductility, have slowed down their emerging commercial applications. While electrochemical preparation of conducting polymers has been shown to be the most satisfactory process from the viewpoint of fundamental investigations, it is likely to be inappropriate for the large-scale industrial production of bulk quantities of these materials. This is particularly true where large molecular entities, e.g., copolymers or different additives, need to be incorporated into conducting polymer matrices in order to obtain tailored performance characteristics of the resulting polymer.
In order to compete with more-advanced interconnect systems, such as hybrid circuits and multichip modules (MCMs), future PWBs will have to be designed so that their size and cost advantages can be used to find a wider range of applications. This will require PWBs with increased conductor density. Increasing the conductor density requires finer lines and spaces ( less than 5 mils), smaller vias ( less than 12 mils), thinner multilayer boards ( less than 0.032 in), and improved insulation resistance will be necessary. Finer lines and pitch will require high-resolution imaging and precision etching. The presence of plated-through-holes (PTHs 0.062-0.04 in) and vias ( less than 0.10 in) in ever-increasing numbers, will present a challenge in laminating, drilling and metallization. Finally, the achievement of these objectives will requires improved electronically conductive polymers, as well as formulations and method for preparing electronically conductive polymers.
Consequently, there remains a need for improved electronically conductive polymers having sufficiently high conductivity to support a practical direct metallization process for preparing electronic circuits. It would be desirable to have improved formulations and methods for preparing electronically conducting polymers that are environmentally friendly, provide improved line definitions, avoid polymer solubility problems, can easily incorporate additives, does not depend upon electroless-copper plating, minimizes hazardous chemicals and copper plating solutions, requires fewer process steps, provides simplified through-hole metallization, and facilitates increased conductor densities.
The present invention provides a formulation for forming an electronically conducting polymer. The formulation contains an electron acceptor comprising a dopant anion and a metal cation selected from Ag+, Fe3+, Cu2+ or combinations thereof. The preferred electron acceptor is a silver salt, preferably selected from AGNO3, AgClO4 and AgNO2. The formulation also contains between 2 and 100 moles of a polymerizable component per mole of the electron acceptor, the polymerizable component being selected from pyrrole, aniline, their oligomers, or combinations thereof The polymerizable component preferably comprises pyrrole and aniline, more preferably the polymerizable component consists essentially of pyrrole and between about 14 and about 18 mole percent aniline. In addition, the formulation includes an aqueous solvent selected from acetonitrile, acetone, and combinations thereof, the aqueous solvent having up to 30 volume percent water. Preferably, the aqueous solvent has between about 1 and about 5 volume percent water.